The present invention provides an improved method for detecting poor hole cleaning and stuck pipe during rotary drilling of a well. The present invention provides an improved method of preventing drilling delays, losses and hazards by early detection of conditions favorable for stuck pipe during rotary drilling of a well.
Wells are generally drilled to recover natural deposits of hydrocarbons and other desirable, naturally occurring, materials trapped in geological formations in the earth""s crust. A slender well is drilled into the ground and directed to the targeted geological location from a drilling rig at the surface. In conventional xe2x80x9crotary drillingxe2x80x9d operations, the drilling rig rotates a drillstring comprised of tubular joints of steel drill pipe connected together to turn a bottom hole assembly (BHA) and a drill bit that is connected to the lower end of the drillstring. During drilling operations, a drilling fluid, commonly referred to as drilling mud, is pumped and circulated down the interior of the drillpipe, through the BHA and the drill bit, and back to the surface in the annulus. It is also well known in the art to utilize a downhole mud-driven motor, located just above the drill bit, that converts hydraulic energy stored in the pressurized drilling mud into mechanical power to rotate the drill bit. The mud circulating pumps that pump the drilling mud and thereby power the mud-driven motor are sealably connected to the surface end of the drillstring through the standpipe and a flexible hose-like connection called a kelly.
When drilling has progressed as far as the drillstring can extend without an additional joint of drillpipe, the mud circulating pumps are deactivated and the end of the drillstring is set in holding slips that support the weight of the drillstring, the BHA and the drill bit. The kelly is then disconnected from the end of the drillstring, an additional joint of drillpipe is threaded and torqued onto the exposed, surface end of the drillstring, and the kelly is then reconnected to the top end of the newly connected joint of drillpipe. Once the connection is made, the mud pumps are reactivated to power the drill motor and drilling resumes.
To isolate porous geologic formations from the wellbore and to prevent collapse of the well, the well is generally cased with tubular steel pipe joints connected together to form a casing string. Casing is set in progressively smaller diameter sections as drilling progresses. Downhole conditions and the physical properties of drilled formations determine when a section of casing must be set in order to isolate exposed wellbore. During drilling operations, the drilling rig extends the drillstring through the casing and into the open wellbore and rotates the drill bit against rock and geologic formations lying in the trajectory of the drilling bit.
The fluid pressure in porous and permeable geologic formations penetrated by the wellbore is generally balanced by the hydrostatic pressure of the column of drilling mud in the well. Pressurized drilling mud is pumped into the surface end of the tubular drillstring by mud pumps that circulate mud down through the interior of the drillstring, through the BHA and drill bit and back up to the surface through the casing/drillstring annulus. Drilling mud is specially designed to not only balance formation pressure, but also to cool and lubricate the drillstring and drill bit, and to suspend and transport drill cuttings to the surface for removal. The process of using drilling mud to suspend and transport cuttings out of the wellbore is often called xe2x80x9chole cleaning.xe2x80x9d
Efficient hole cleaning greatly benefits the overall drilling process. A smooth and uniform flow of drilling mud promotes easy and cost-effective drilling. It is desirable for the cuttings to be uniformly dispersed and suspended in the flowing drilling mud as they are carried to the surface through the annulus. The flow rate, flow regime and viscosity of the drilling mud are key factors that determine the capacity of the drilling mud to suspend and transport drill cuttings to the surface. Slender, intermediate deviations (40xc2x0-60xc2x0) and horizontal wellbores are more subject to poor hole cleaning and stuck pipe than are larger, vertical wells because drill cuttings settling out of drilling mud tend to accumulate on the lower or downward side of the well. The unwanted accumulation of a stationary bed of drill cuttings interferes with the drilling process by resisting reciprocation and rotation of the drillstring. Poor hole cleaning results in high torque (resistance to rotation) and excessive drag (resistance to reciprocation) on the drill string, hole pack-off (resistance to drilling mud circulation) and, ultimately, stuck pipe. These conditions may cause well control problems, delays in drilling and poor drilling efficiency, adversely impacting the well economics and possibly resulting in the equipment loss or damage or even a loss of the wellbore.
A method has been devised for early detection of poor hole cleaning and stuck pipe using measured wellbore data. U.S. Pat. No. 5,454,436, issued to Jardine et al., describes a method of diagnosing and warning of pipe sticking during drilling operations and is incorporated herein by reference. The Jardine method mathematically analyzes the standpipe pressure (SPP) trace and the surface torque trace comprising a series of standpipe drilling mud pressures and surface torque measurements over the same time period, respectively. The input SPP trace and surface torque trace can be seen in FIGS. 1(A) and 1(B), respectively. Jardine""s method determines the SPP skew of the SPP trace and the normalized standard deviation of the surface torque trace as shown in FIGS. 2(A) and 2(B), respectively. This attenuates and enables correlation of increases in the SPP and surface drillstring torque that are characteristic signatures of accumulated drill cuttings obstructing mud flow and packing off around the drill string. Jardine""s method then determines the product of the SPP skew and the normalized standard deviation of the drill string torque trace to further attenuate the data to indicate events causing simultaneous spikes in the SPP skew and the surface torque normalized standard deviation as shown in FIG. 3(A). Finally, Jardine""s method integrates the product of the SPP skew and the normalized standard deviation of the surface torque to produce the diagnostic shown in FIG. 3(B). The integrated value is a more reliable diagnostic than the product because the skew should oscillate between positive and negative values for normal drilling conditions, in other words, pressure fluctuations will be both positive and negative, and hence the integral should be close to zero. However, the integrated value will exhibit an increasing positive trend in the presence of positive pressure fluctuations indicative of poor hole cleaning or stuck pipe. Trend analysis or a simple thresholding technique can then be used to identify when this positive trend occurs.
The method disclosed by Jardine is, however, hindered by extraneous influences (besides poor hole cleaning) that contribute to the SPP trace, and therefore interfere with detection of poor hole cleaning and retard the accuracy of the wellbore diagnosis.
What is needed is a method of detecting poor hole cleaning or conditions favorable for the occurrence of stuck pipe that is not hindered by extraneous influences. What is needed is a method of detecting poor hole cleaning or conditions favorable for the occurrence of stuck pipe using data that is already generally available on drilling rigs, or with reliable and inexpensive additional downhole equipment. What is needed is a method of raising an alarm at the onset of poor hole cleaning or stuck pipe to alert persons operating the drilling rig to take timely remedial measures.
The present invention provides a method for early detection of poor hole cleaning or conditions favorable for the onset of stuck pipe during rotary drilling. The method provides early detection by inventive analysis and use of drill string torque data and downhole annular fluid pressure data, preferably on a real-time or near real-time basis. The annular fluid pressure is continuously measured downhole at the BHA (and possibly other depths) and communicated to the surface using telemetry, and is correlated with either surface or downhole torque measurements to attenuate certain signature responses. The method enables drilling rig operators to observe and recognize attenuated signature responses in downhole annular fluid pressure and surface or downhole torque data that arise from poor hole cleaning or stuck pipe in time to take preventive and remedial measures. The method uses generally available data to prevent the unwanted delays, hazards and losses that result from poor hole cleaning and stuck pipe.
Downhole annular fluid pressure is typically measured by the bottom hole assembly (BHA) and communicated to the surface during periods of active mud circulation. At the surface, the measured downhole annular fluid pressure trace is analyzed along with a simultaneously measured trace of the surface torque applied to rotate the drillstring. This correlation, enabled by mathematical manipulation of the data, enables the drilling rig operator to detect recognizable responses characteristic of poor hole cleaning and stuck pipe.
The downhole pressure trace commonly available to facilitate use of the improved method is measured at the BHA and communicated to the surface using telemetry, preferably mud-pulse telemetry. The telemetry data capacity of the drilling mud may allow additional downhole devices to transmit additional data to the surface. Optionally, the method may utilize additional downhole pressure traces or other data measured at instruments and sensors strategically placed along intervals of interest in the drillstring. The method may use correlation of one xe2x80x9clocalxe2x80x9d annular fluid pressure trace to others measured at the BHA or other depths to diagnose the exact location and nature of poor hole cleaning or stuck pipe.
Optionally, the method may comprise correlating measured downhole drill string torque with the measured downhole annular fluid pressure trace(s).